In the art of rotary piston engines, different solutions for achieving tightness, that is, sealing, of the piston against the enclosing casing wall during the course of movement are known. So-called rotor segment engines achieve an almost good tightness due to the high size accuracy of the components rotor, casing and blades which surround the operating space and yield the smallest possible gap between the components. In certain cases the tightness can even be improved by introducing a suitable fluid into the engine, causing a small fluid film to act as a sealing body between the components. When doing compression work with such engines gap losses must be accounted for. Such losses result in a reduction of the delivery output which can be compensated for by increasing the driving power of the compressor. In expansion engines the gap losses may lead to a loss in operation, especially when a damaging expansion takes place mainly via the gaps which result in providing ineffective rotary power of the rotor.
On the other hand, expanding media in high temperature conditions such as present in thermal engines can lead to a destruction of the engine as passing hot gases cause material erosion to component parts thereby increasing the gaps.
F. Wankel found that rotary combustion engines having more than three components moving in relation to each other, such as a rotor, movable piston parts fitted at the rotor, and casing, cannot function, as the sealing elements cannot be arranged such that during the course of motion of the engine, a unified spatial system of sealing lines having the same geometrical shape can be achieved. This defect is clearly visible in rotor segment engines. Though it may be possible to achieve a radial and axial tightness against the casing wall by spring sealing strips along the blade edges, the sealing line is interrupted in the area of the rotor hub by a remaining unsteadiness, which will lead to an untightness of the engine. Resulting from this experience, the rotary piston internal combustion engine developed by F. Wankel was an engine type having only 2 components moving in relation to each other and enclosing the working space: a casing with a trochoidal running way and a rotary piston also derived from a trochoid as internal enclosing body of the casing running way. Sealing strips can be fitted on this piston fulfilling the conditions of an unchanged geometrical shape. This type of engine has become known as Wankel engine.
In spite of the advantages and the successful development of this type of engine, certain technological targets could not be reached. The geometrically determined change in volume of the trochoid does not allow carrying out a traditional Diesel process. Though less important, the lubrication of the sealing strips and, connected with it, the heat dissipation from the piston to the casing wall are also concerns.